1. Field of the Invention
This invention relates generally to scientific and industrial cameras, and more particularly, to a highly controllable video camera that utilizes a charge coupled device (CCD) image sensor, or other solid state image sensor, configured in either a full or partial exposure mode, in combination with an electronic memory to produce an electronic representation of an object for transmission to a video display device or a digitally responsive device.
2. Brief Description of the Prior Art
The use of CCD or solid state imaging devices in place of film in cameras is well known, as is shown in Ochi et al, U.S. Pat. No. 4,541,016, issued Sept. 10, 1985. In general, CCD cameras are comprised of optical lenses that focus light from an object on to a CCD, whereupon the light is converted into electrical charges or electrical signals that are transferred from the CCD for display on a video unit.
A CCD is comprised of a pattern of closely spaced light sensitive image collecting elements or electrodes disposed on, and electrically isolated from, the surface of a semiconductor material. When proper voltages are applied to the electrodes, potential wells, capable of holding and transferring separate charges are formed under the electrodes. When the semiconductor material is exposed to light, near where such a potential well is formed, charge carriers generated by the light are collected and held in the potential well.
The charge collected in each potential well is representative of a pixel of information related to the light from the object focussed on the CCD. A typical CCD may be comprised of as many as 576 horizontal lines of 386 electrodes, more or less. However, as will be seen below, only about half of these number of horizontal lines of electrodes are used for imaging purposes by prior art frame transfer CCDs. The electrodes of the CCD are typically separated in a three-phase electrode structure, as is shown by Bixby, U.S. Pat. No. 4,338,514, issued July 6, 1982, but other electrode structures, such as two-phase or four-phase, could be adapted to achieve the same results. To transfer charges out of the CCD, adjacent electrodes are sequentially voltage pulsed between high and low levels, thereby line-by-line parallel shifting rows of charges to a read-out register within the CCD. As each row is transferred into the read-out register of the CCD, each pixel of information in that horizontal row is serially transferred from the CCD.
To create an interlaced picture using prior art frame transfer devices, as is required for TV or video display, charges are first collected under only one of the phase electrodes in the image section, after collection, the charges are shifted into a separate temporary store (which forms the remainder of electrodes in the CCD and reduces the available imaging area by at least half) and then shifted line-by-line from the store for output from the CCD. When the CCD is emptied, the drive pulse sequence of the image section is then modified so that the remaining portion of the image can be collected under the second and third phase electrodes, at which point these additional charges are transferred from the CCD. The first charge collecting period, or exposure, will form a first odd field of the interlaced image and the second charge collecting period will form a second even field of the interlaced image, as is required for video display. Thus, each image produced by prior art cameras requires two separate exposures (an odd and even field) of the CCD in order to recreate an interlaced TV signal, while requiring half of the imaging area of the CCD for non-imaging purposes.
Other applications have attempted to recreate the interlaced signal by increasing the image area of the CCD, so that an image passed across a frame transfer type CCD can be imaged by the CCD a number of times during each exposure, such as in Dillon et al, U.S. Pat. No. 4,338,634, issued July 6, 1982. However, such applications require a larger CCD imaging area, and the transfer timing for the electrodes must be synchronized with the speed of the image, giving such applications very limited utility.
To fully utilize the high-speed transfer characteristics of a CCD imaging camera, a wide variety of mechanical or electromechanical shutters and diaphragms have been developed. Prior art CCD cameras utilizing such devices have been able to achieve shutter speeds as high as 12,000 frames per second (with split-screen playback) or one frame approximately every eighty-three microseconds. See, Hyzer, The Spin Physics Sp-2000 Motion Analysis Systems, PHOTOMETHODS, May 1981. See also, Lloyd et al, U.S. Pat. No. 4,131,919, issued in December of 1978 disclosing an electronic still camera wherein the image and light exposure of the CCD array is controlled by a conventional photo-electric circuit which also controls the diaphragm aperture positioned in front of the array.
At faster frame rates, prior art exposure control devices have been found to be incapable of providing sufficient light (by means of opening the diaphragm) to match the shutter speed. To overcome this problem, a strobe light has been used in synchronism with the shutter to increase available light during an exposure period. However, strobe lights, as well as other light sources, can not be sufficiently controlled so as to inhibit incident light from reaching the CCD and being converted into unwanted information that is transferred along with wanted information as noise. Incident light noise, along with any charge left over in the electrodes during transfer, is known as smearing or blooming. Various devices have been developed to suppress smearing/blooming, such as is shown in Levine, or Akiyama, U.S. Pat. No. 4,233,632, issued Nov. 11, 1980.